High density polyethylene packaging

ABSTRACT

High density polyethylene made using a cobalt or iron complex of a selected tridentate ligand as a polymerization catalyst may be made into packaging which has advantageous properties, especially lower permeation to ambient materials such as oxygen and/or water. The packaging, such as bottles, bags and rigid storage tanks, may be formed by conventional methods.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 from U.S.Provisional Application Ser. No. 60/151,916 (filed Sep. 1, 1999), whichis incorporated by reference herein as if fully set forth.

FIELD OF THE INVENTION

High density polyethylene (HDPE) made using certain late transitionmetal containing catalysts has lower water vapor and/or oxygentransmission rates than similar HDPEs made using other polymerizationcatalysts, thereby making them superior in uses, such as packaging,where lower water vapor and/or oxygen permeation rates are advantageous.

TECHNICAL BACKGROUND

High density polyethylene (HDPE) is an important commercial product,large quantities being produced worldwide. HDPE is typically recognized(and is defined for the purposes of the present invention) as asubstantially linear, semi-crystalline, polymer of ethylene (preferablya homo-polymer but also on occasion containing very minor amounts ofother well-known comonomers), possessing a density of 0.94 g/mL orhigher.

An important use of HDPE is in packaging, which may be divided into twogeneral types—rigid packaging such as bottles and tanks, and flexiblepackaging such as bags and pouches. The former may be formed by suchmethods as blow or injection molding, and the latter are usually formedfrom films having one or more layers, at least one of which is HDPE.

HDPE is a favored packaging material for many products because of lowcost, relatively easy formability and good toughness, and for someproducts having low permeation rates for certain materials eitherdeleterious to these products, or to keep the package's contents fromdiffusing from the package and being lost, such as water and/or oxygen.Among the types of products where these low permeation rates areimportant are foods, both dry and liquid materials, and lubricatingoils. For example for dry foods low water vapor transmission rates areimportant to keep the foods crisp, while low oxygen transmission ratesare important for any foods that may oxidize, forming off colors and/ortastes or smells in the food. The lower the transmission rates of thepackaging, the better the food will taste and/or look, and/or the longerthe food may be stored before being used, and/or the thickness of thepackaging may be reduced without deleteriously affecting absolute ratesof transmission, all of course important advantages. In some instances,such as bottles for toiletries such as perfume or cologne, it may bedesirable to keep water in and/or oxygen out. Other combinations will beobvious to the artisan.

U.S. Pat. No. 5,955,555, WO99/12981, WO99/46302, WO99/46303, WO99/46304,WO99/46308, WO99/62963 (corresponding to U.S. patent application Ser.No. 09/317,104, filed May 21, 1999 now U.S. Pat. No. 6,252,022),WO00/15646, WO00/24788, WO00/32641, G. J. P. Brito-vesk, et al., J.Chem. Soc., Chem. Commun., p. 849-850 (1998), and B. L. Small, et al.,Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) vol. 39, p. 213 (1998)(all incorporated by reference herein for all purposes as if fully setforth), all report the polymerization of ethylene using iron and cobaltcomplexes of certain tridentate ligands. No mention is made of the useof the resulting polymers for packaging where improved (lower) watervapor and/or oxygen permeation rates of HPDE are of interest.

SUMMARY OF THE INVENTION

Disclosed herein is a package comprising a high density polyethyleneobtainable (and preferably obtained) by polymerizing ethylene in thepresence of a polymerization catalyst component which comprises an ironor cobalt complex of a compound of the formula (I)

wherein:

R¹, R², R³, R⁴ and R⁵ are each independently selected from the groupconsisting of hydrogen, a hydrocarbyl, an inert functional group and asubstituted hydrocarbyl; and

R⁶ and R⁷ are each independently selected from the group consisting ofaryl and substituted aryl.

This invention also concerns a process for making a package, comprisingthe steps of:

(a) polymerizing ethylene in the presence of a polymerization catalystcomponent to form high density polyethylene, the polymerization catalystcomponent comprising an iron or cobalt complex of a compound of theformula

wherein:

R¹, R², R³, R⁴ and R⁵ are each independently selected from the groupconsisting of hydrogen, hydrocarbyl, an inert functional group orsubstituted hydrocarbyl; and

R⁶ and R⁷ are aryl or substituted aryl; and

(b) forming said polyethylene into said package.

Preferably the package referred to above is a rigid storage tank or isotherwise based upon a multilayer sheet or film containing at least onelayer of, or in which at least one of the layers comprises, the HDPEdefined above.

This invention further concerns a process for lowering the water vaporand/or oxygen transmission rates of a package manufactured at least inpart with a first HDPE, comprising the step of replacing, during themanufacture of said package, at least a portion of the first HDPE with asecond HDPE obtainable (and preferably obtained) by polymerizingethylene in the presence of a polymerization catalyst component whichcomprises an iron or cobalt complex of a compound of the formula (I)

wherein:

R¹, R², R³, R⁴ and R⁵ are each independently selected from the groupconsisting of hydrogen, a hydrocarbyl, an inert functional group and asubstituted hydrocarbyl; and

R⁶ and R⁷ are each independently selected from the group consisting ofaryl and substituted aryl.

This invention still further concerns a process for lowering the watervapor and/or oxygen transmission rates of a package manufactured fromone or more layers of a first HDPE, comprising the step of replacing,during the manufacture of said package, at least a portion of at leastone of the layers of the first HDPE with a layer of a second HDPEobtainable (and preferably obtained) by polymerizing ethylene in thepresence of a polymerization catalyst component which comprises an ironor cobalt complex of a compound of the formula (I)

wherein:

R¹, R², R³, R⁴ and R⁵ are each independently selected from the groupconsisting of hydrogen, a hydrocarbyl, an inert functional group and asubstituted hydrocarbyl; and

R⁶ and R⁷ are each independently selected from the group consisting ofaryl and substituted aryl.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Herein certain terms are used which are defined below.

A “hydrocarbyl group” is a univalent group containing only carbon andhydrogen. As examples of hydrocarbyls may be mentioned unsubstitutedalkyls, cycloalkyls and aryls. If not otherwise stated, it is preferredthat hydrocarbyl groups (and alkyl groups) herein contain 1 to about 30carbon atoms.

By “substituted hydrocarbyl” herein is meant a hydrocarbyl group thatcontains one or more substituent groups which are inert under theprocess conditions to which the compound containing these groups issubjected (e.g., an inert functional group, see below). The substituentgroups also do not substantially detrimentally interfere with thepolymerization process or operation of the polymerization catalystsystem. If not otherwise stated, it is preferred that substitutedhydrocarbyl groups herein contain 1 to about 30 carbon atoms. Includedin the meaning of “substituted” are rings containing one or moreheteroatoms, such as nitrogen, oxygen and/or sulfur. In a substitutedhydrocarbyl, all of the hydrogens may be substituted, as intrifluoromethyl.

By “(inert) functional group” herein is meant a group, other thanhydrocarbyl or substituted hydrocarbyl, which is inert under the processconditions to which the compound containing the group is subjected. Thefunctional groups also do not substantially deleteriously interfere withany process described herein that the compound in which they are presentmay take part in. Examples of functional groups include halo (fluoro,chloro, bromo and iodo), and ether such as —OR⁵⁰ wherein R⁵⁰ ishydrocarbyl or substituted hydrocarbyl. In cases in which the functionalgroup may be near a transition metal atom, the functional group aloneshould not coordinate to the metal atom more strongly than the groups inthose compounds that are shown as coordinating to the metal atom, thatis they should not displace the desired coordinating group.

By a “cocatalyst” or a “catalyst activator” is meant one or morecompounds that react with a transition metal compound to form anactivated catalyst species. One such catalyst activator is an “alkylaluminum compound” which, herein, is meant a compound in which at leastone alkyl group is bound to an aluminum atom. Other groups such as, forexample, alkoxide, hydride and halogen may also be bound to aluminumatoms in the compound.

By “aryl” is meant a monovalent aromatic group in which the free valenceis to the carbon atom of an aromatic ring. An aryl may have one or morearomatic rings which may be fused, connected by single bonds or othergroups.

By “substituted aryl” is meant a monovalent aromatic group substitutedas set forth in the above definition of “substituted hydrocarbyl”.Similar to an aryl, a substituted aryl may have one or more aromaticrings which may be fused, connected by single bonds or other groups;however, when the substituted aryl has a heteroaromatic ring, the freevalence in the substituted aryl group can be to a heteroatom (such asnitrogen) of the heteroaromatic ring instead of a carbon.

By “relatively noncoordinating” (or “weakly coordinating”) anions aremeant those anions as are generally referred to in the art in thismanner, and the coordinating ability of such anions is known and hasbeen discussed in the literature, see for instance W. Beck., et al.,Chem. Rev., vol. 88 p. 1405-1421 (1988), and S. H. Stares, Chem. Rev.,vol. 93, p. 927-942 (1993), both of which are hereby included byreference. Among such anions are those formed from aluminum compoundssuch as those described in the immediately subsequent paragraph and X⁻,including (R⁵¹)₃AlX⁻, (R⁵¹)₂AlClX⁻, R⁵¹AlCl₂X⁻, and R⁵¹AlOX⁻, whereinR⁵¹ is alkyl. Other useful noncoordinating anions include BAF⁻{BAF=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate}, SbF₆ ⁻, PF₆ ⁻, andBF₄ ⁻, trifluoromethanesulfonate, p-toluenesulfonate, (R_(f)SO₂)₂N⁻, and(C₆F₆)₄B⁻.

By “neutral Lewis base” is meant a compound, which is not an ion, whichcan act as a Lewis base. Examples of such compounds include ethers,amines, sulfides, and organic nitriles.

By “cationic Lewis acid” is meant a cation that can act as a Lewis acid.Examples of such cations are sodium and silver cations.

By a “package” is meant any container that is meant to be sealed most ofthe time (sometimes called “protective packaging”), especially beforethe contents are used, against ambient conditions such as air and/ormoisture, and/or loss of the package's content as by evaporation. Thepackage may be designed so that the seal against ambient conditions maybe broken permanently broken as by cutting or tearing to open a sealedbag, or may be broken temporarily, as by opening a screw-cap bottle andthen replacing the cap. The package may have one or more inlets and/oroutlets to store a material that may be added to and/or withdrawn fromthe package without further opening the package. The package may beformed in any manner (see below).

The polyethylene used herein is obtainable, and preferably obtained, bypolymerizing ethylene in the presence of a catalyst component comprisingan iron or cobalt complex of (I).

Such iron and cobalt complexes may be formed by a variety of methods,for example, the complex may be formed before the polymerization isperformed, with the preformed complex being added to the polymerizationprocess, or the complex may be formed in situ in the polymerizationprocess, such as disclosed in previously incorporated U.S. Pat. No.5,955,555 and WO99/12981, or in WO99/50273 (corresponding to U.S. patentapplication Ser. No. 09/277,910, filed Mar. 29, 1999) and WO00/08034(also incorporated by reference herein for all purposes as if fully setforth), and reference may be had thereto for further details regardingthese catalyst complexes and the preparation thereof.

In preferred embodiments, the catalyst complexes are referred to hereinby the formula (I)MX_(n), wherein (I) is the compound (I), M is selectedfrom the group consisting of Fe and Co, n is 2 or 3, and each X isindependently an anion. Preferably each X is independently a halide, andmore preferably chloride. This formula can be depicted by the structure(II) below:

wherein R¹, R², R³, R⁴, R⁵, R⁶ and R⁷ are as defined above, and n is 2or 3.

The following are preferred embodiments of (I):

R¹, R² and R³ are hydrogen; and/or

R⁴ and R⁵ are each independently selected from the group consisting ofhydrogen and methyl, and both are more preferably methyl; and/or

R⁶ and R⁷ are each independently selected from aryl, andhalo-substituted aryl, and more preferably aryl and halo-substitutedphenyl, and especially aryl.

In (I) and (II), it is preferred that R⁶ is

and R⁷ is

wherein:

R⁹, R¹⁰, R¹¹, R¹⁴, R¹⁵, R¹⁶, R¹⁹, R²⁰, R²¹ and R²² are eachindependently hydrocarbyl, substituted hydrocarbyl or an inertfunctional group; provided that any two of R⁹, R¹⁰, R¹¹, R¹⁴, R¹⁵, R¹⁶,R¹⁹, R²⁰, R²¹ and R²² vicinal to one another taken together may form aring.

In particularly preferred embodiments, R⁹, R¹¹, R¹⁴ and R¹⁶ arehydrogen; R¹⁹, R²⁰, R²¹ and R²² are independently methyl, ethyl, propyl,isopropyl, halo or trihalomethyl; and R¹⁰ and R¹⁵ are independentlyhydrogen, methyl, ethyl, propyl, isopropyl, halo or trihalomethyl.

The compound (I) is preferably formed by contacting a2,6-diacetylpyridine with an amino-substituted aryl compound underconditions so as to form imine linkages, as disclosed in previouslyincorporated such as disclosed in previously incorporated U.S. Pat. No.5,955,555, WO99/12981, WO99/50273 (corresponding to U.S. patentapplication Ser. No. 09/277,910, filed Mar. 29, 1999 now U.S. Pat. No.6,232,259 and WO00/08034, and reference may be had thereto for furtherdetails.

The polymerization catalyst component may optionally comprise othercomponents such as, for example, co-catalysts and catalyst activators.

For example, the catalyst component may contain a compound W, which is aneutral Lewis acid capable of abstracting an X⁻ from the catalystcomplex to form WX⁻, provided that the anion formed is a weaklycoordinating anion, or a cationic Lewis or Bronsted acid whosecounterion is a weakly coordinating anion; and provided that when noneof X is alkyl, acyl or hydride, said second compound is capable oftransferring hydride or alkyl to M.

Preferred compounds W include alkylaluminum compounds and/or otherneutral Lewis bases. For example, the complex (I)MX_(n) wherein M is Coor Fe, n is 2 or 3, and X is halide, may be contacted with analkylaluminum compound such as methylaluminoxane to form a highly activepolymerization catalyst. Iron complexes are preferred, Fe[II] andFe[III] complexes are more preferred, and Fe[II] complexes areespecially preferred. A particularly preferred complex is[2,6-diacetylpyridinebis{(2,4,6-trimethyl)phenylimine}]iron dichloride,especially in combination with methylaluminoxane.

Additional details may again be found in previously incorporated U.S.Pat. No. 5,955,555 and WO99/12981, or in WO99/50273 (corresponding toU.S. patent application Ser. No. 09/277,910, filed Mar. 29, 1998 nowU.S. Pat. No. 6,232,259) and WO00/08034.

The polymerization to form the HDPE used in the present invention may becarried out in the gas phase, or in the liquid phase especially inslurry. In one preferred method the catalyst system, especially the ironor cobalt complex, is supported on a solid (heterogeneous) support, suchas silica, alumina, another polymer, or a metal halide. Thepolymerization may also be carried out in batch, semi-batch, continuous,or continuous polymerizations in series. Continuous polymerizations arepreferred.

Useful and preferred systems and conditions for carrying out theethylene polymerization are described in, for example, previouslyincorporated U.S. Pat. No. 5,955,555, WO99/12981, WO99/46302,WO99/46303, WO99/46304, WO99/46308, WO99/62963 (corresponding to U.S.patent application Ser. No. 09/317,104, filed May 21, 1999 now U.S. Pat.No. 6,252,022), WO00/15646, WO00/24788, WO00/32641, and reference may behad thereto for further details.

The package may be formed by any conventional method for formingpackages from thermoplastics. For rigid containers such as bottles,sealable cartons, storage tanks especially for water, chemicals, fuelsand solvents, cosmetic jars, barrels, and drums, the container may beblow molded or rotomolded. In extrusion blow molding for example thebody of the container such as a bottle may just be the HDPE describedherein, or may contain two or more layers, at least one of which is theHDPE described herein. The cap of a screw-cap or snap-cap bottle be madeof the HDPE, as by injection molding, or thermoforming, or may be madeof another material, such as another thermoplastic, a thermosetingresin, metal, etc., as appropriate. Two halves (or other fractionalparts) of the container may be thermoformed from a sheet of the HDPE ora multilayer sheet containing at least one layer of the HDPE, and theparts joined by welding or by an adhesive or both.

Another form of packaging is flexible bags or pouches, many of which areheat sealed or sealed with adhesive after being filled with whateverthey are to contain. These may be made from films or tubes of the HDPE,or multilayer films or tubes that contain at least one layer of theHDPE. A tube for example may be heat sealed at one end, filled withliquid or solid, and then heat sealed or sealed with adhesive at theother end. A film may be folded once, heat sealed along the sides,filled, and then heat sealed at the other end, or adhesive may be usedfor some of all of the seals. Pouches may be formed by similar methods,except they tend to be more rigid, because of the thickness of thematerial from which they are made and/or their configuration. Theflexible packages may also be sealed by interlocking seals that may beopened and resealed, sometimes called zip lip seals.

All of the above types of packages, and other types of packages, may bemade by methods known in the art for making packaging fromthermoplastics, see for example H. Mark, et al., Ed., Encyclopedia ofPolymer Science and Engineering, Vol. 10, John Wiley & Sons, New York,1987, p. 684-720, which is hereby included by reference.

As such, HDPE-containing packaging having improved (lower) water vaporand/or oxygen transmission rates can be prepared by replacing, duringthe manufacture of the package, at least a portion of the HDPE with asecond HDPE obtainable (and preferably obtained) as set forth above.

Replacement can be accomplished by substituting this second HDPE at thetime of manufacture of the package. The second HDPE can simply be usedin place of conventional HDPE, or can be blended with conventional HDPEin order to replace a portion of the same. For multilayer packaging asdescribed above, one or more of the layers can be prepared solely fromthe second HDPE, or a blend of the second HDPE with conventional HDPE,then substituted for a layer of conventional HDPE.

The blend can be a standard physical blend, melt blend, or even areactor blend prepared by polymerizing ethylene in the presence of thecatalyst complex referred to above, along with a second active HDPEcatalyst (co-catalyst) such as a Ziegler-Natta and/or metallocene-typecatalyst known in the art. See, for example, previously incorporatedWO99/12981, WO99/46302, WO99/46303, WO99/46304, WO99/46308, WO00/15646,WO00/24788, WO00/32641, as well as WO98/38228 and WO99/50318(corresponding to U.S. patent application Ser. No. 09/619,509, filedJul. 19, 2000), which are also incorporated by reference herein for allpurposes as if fully set forth.

In the Examples the following tests were used:

Oxygen transmission was measured using an Oxtran® 2/20 Model T, hightransmission rate tester (Mocon, Inc., Minneapolis, Minn. 55428 U.S.A.)at 23° C. and 0% relative humidity using 100% oxygen (not air). Samplefilms were run in duplicate. The theory of the test is outlined in ASTMD3985-81 “Standard test Method for Oxygen Gas Transmission Rate throughplastic film and sheeting using coulometric sensor”. Results arereported per 25 μm thickness (1 mil). Values were corrected to abarometric pressure of 101 kPa (760 mmHg). Samples were conditioned for2 h prior to testing. Test area was 100 cm². Examine time was 60 min.Nitrogen gas flow was 20.1 smLm.

Moisture Vapor Transmission was measured using a Permatran® W 3/31,water vapor transmission system (Mocon, Inc.) at 38° C. and 90-100% RH.Sample films were run in duplicate. The theory of the test is outlinedin ASTM D1249-90 “Standard Test Method for Water Vapor Transmissionthrough plastic film and sheeting using a modulated infrared sensor”.Results are reported per 25 μm (1 mil) thickness. Sample test area was50 cm². Relative Humidity was essentially 100%. Sample was conditioned 2h before testing. Examine time was 30 min. Test temperature was 37.8° C.

Unless otherwise noted polymer density was measured on samples preparedaccording to ASTM D1928, Procedure C, slightly modified.Polytetrafluoroethylene coated aluminum foil was used as the partingsheets, and the heatup time was 1.5 min at 180° C., while eliminatingthe backup sheets and placing the sandwich of material and a 10 mil (250μm) chase of Teflon® FEP (available from E. I. DuPont de Nemours & Co.,Wilmington, Del. 19898) between the two sheets of foil and placing itdirectly between the press platens. The density measurement was donefollowing ASTM D1505. Density of extruded blown films was measured onthe as made films using the method of ASTM D1505.

Melting point and heat of fusion of polymers was measured bydifferential scanning calorimetry, using values from the 2nd heat, andusing a heating rate of 10° C./min. The peak of the melting endothermwas taken as the melting point.

I₂ and I₁₀, which are melt indices at different shear stresses, weremeasured by ASTM method D1238.

In the Examples, all pressures are gauge pressures.

In the Examples Mn is number average molecular weight, Mw is weightaverage molecular weight, both determined by Gel PermeationChromatography, PD is Mw/Mn, Tm is melting point, and ΔH_(f) is heat offusion.

EXAMPLE 1

(III) can be made, for example, by procedures described in previouslyincorporated U.S. Pat. No. 5,955,555 and WO99/12981. Inside a nitrogenpurged drybox, (III) (28.0 mg, purified by recrystallization fromCH₂Cl₂/pentane) was placed in anhydrous toluene (˜10 ml) and MMAO-12(1.2 g, Akzo-Nobel, 12.25 wt % modified (contains some isobutyl groups)MAO in toluene) was added. The resulting orange solution was shaken for1 min and then dehydrated silica (1.9 g, Grace-Davison 948 sphericalsilica) was added and the mixture shaken for 1 h. The orange solid wasfiltered from the clear solution, washed well with anhydrous toluene andfinally with anhydrous pentane and dried under vacuum. InductivelyCoupled Plasma analysis indicated: Al=6.7%, Fe=0.12%.

In a dry box, small stainless cylinders (25 to 40 mL volume) werecharged with supported (II) (150 mg total weight including support) anda solution of triisobutyl aluminium (5 mL of a 1 M solution in hexane,Aldrich). The cylinders were connected to the autoclave reactor portsunder nitrogen purge of the connections. Cylinder pressurization lineswere also connected under purge.

Isobutane (1200 g, Matheson C.P. grade) was transferred into a cooledautoclave (−30° C.) by pressure difference. Once the transfer wascompleted, the autoclave [Autoclave Engineers, 1-gal (3.8 L)] was heatedto 20° C. and stirred at 1000 rpm. The solvent was saturated withhydrogen to the designated H₂ pressure. After saturation, the reactorwas heated to 80° C. and pressurized with ethylene to 1.72 MPa. Thetriisobutyl aluminum solution was pushed into the reactor with ethylenefollowed by the catalyst also pushed with ethylene. The final reactorpressure was 2.41 MPa and the ethylene feed was switched from the feedvessels to the side port in the autoclave. The reaction was run for morethan 2 h or until the weight loss in the ethylene cylinder was equal toapproximately 800 g. At the end of the polymerization, the reactor wasvented slowly through a 5 h period, followed by a nitrogen purge priorto opening of the reactor. The polymer was dried overnight. Two polymersamples were prepared at different hydrogen pressures, and their as madeproperties are given below:

H₂ Pressure Density Tm ΔH_(f) Ex. (kPa) I₂ I₁₀ I₁₀/I₂ Mn Mw PD g/mL ° C.J/g 1 640 1.00 15.5 15.5 10330 128267 12.4 0.9641 134.9 242 2 690 1.5322.8 15 9545 109403 11.5

EXAMPLES 3-4 Comparative Examples A-B

The powder polymers from Examples 1 and 2 were each powder blended with500 ppm of Irganox® 1076 and 500 ppm of Irgafos® 168 antioxidants (madeby Ciba-Geigy Corp.).

Alathon® 7820 high density polyethylene was obtained from EquistarChemicals, L.P., Houston, Tex. 77252 U.S.A.).

Sclair® 19C high density polyethylene was obtained from Nova ChemicalsCorp. (Calgary, AB, Canada T2P 5C6), and is believed to have been madein solution using a Ziegler-Natta-type catalyst.

Properties of the polymers used are shown below in Table 1.

TABLE 1 Density, Polymer g/mL I₂ I₁₀ I₁₀/I₂ Ex. 1 0.99 14.59 14.7 Ex. 21.88 24.89 13.2 Alathon ® 7820 0.9563 0.44 8.03 18.4 Sclair ® 19C 0.95230.97 8.85 9.13

Each polymer was blown into nominally 50 μm (2 mil) thick film [actualthicknesses 44.5-50.8 μm (1.75-2.00 mil)] using a 2.54 cm (1″) diameterKillion blown film die fed from a 1.9 cm (¾″) diameter Killion 30/1 L/Dextruder fitted with a 3/1 compression ratio general purpose screw, 0.46cm (0.18″) deep at the feed end, and included mixing heat. The die gapwas 0.11 cm (0.045″). The rear zone of the extruder was at 172-184° C.All other barrel and die temperatures were 220-240° C. The extruderscrew speed was 100 rpm. For the film die, blow-up ratio was 3.5:1, andhaul off speed was 305-335 cm/min (10-11 ft./min). The frost line waskept at about 12.7-14.0 cm (5.0-5.5 inches) above the die, by adjustmentof the blower speed. Properties of the as made films are given in Table2.

TABLE 2 mL-mil g H₂O- Film O₂/100 in²/day mil/100 in²/day Ex. PolymerDensity (nmol/m² · s/25 μm) (μmol/m² · s/25 μm) A Alathon ® 0.949 168.2,167.5 0.27, 0.27 7820 (1346, 1340) (2.7, 2.7) B Sclair ® 19C 0.947178.3, 184.9 0.29, 0.28 (1426, 1479) (2.9, 2.8) 3 Ex. 1 0.955 142.4,143.1 0.22, 0.22 (1139, 1145) (2.2, 2.2) 4 Ex. 2 0.956 142.5, 116.80.19, 0.20 (1140, 934)  (1.9, 2.0)

What is claimed is:
 1. A rigid storage tank comprising a high densitypolyethylene obtained by polymerizing ethylene in the presence of apolymerization catalyst component which comprises an iron or cobaltcomplex of a compound of the formula (I)

wherein: R¹, R², R³, R⁴ and R⁵ are each independently selected from thegroup consisting of hydrogen, a hydrocarbyl, an inert functional groupand a substituted hydrocarbyl; and R⁶ and R⁷ are each independentlyselected from the group consisting of aryl and substituted aryl.
 2. Therigid storage tank as recited in claim 1 wherein said complex is[2,6-diacetylpyridinebis{(2,4,6-trimethyl)phenylimine}]iron dichloride.3. The rigid storage tank as recited in claim 1, characterized in thatsaid high density polyethylene is obtained by polymerizing ethylene inthe presence of said polymerization catalyst component.
 4. A process formaking a rigid storage tank, comprising the steps of: (a) polymerizingethylene in the presence of a polymerization catalyst component to formhigh density polyethylene, the polymerization catalyst componentcomprising an iron or cobalt complex of a compound of the formula

wherein: R¹, R², R³, R⁴ and R⁵ are each independently selected from thegroup consisting of hydrogen, hydrocarbyl, an inert functional group orsubstituted hydrocarbyl; and R⁶ and R⁷ are aryl or substituted aryl; (b)forming said high density polyethylene into said rigid storage tank. 5.The process as recited in claim 4 wherein said complex is[2,6-diacetylpyridinebis{(2,4,6-trimethyl)-phenylimine}]iron dichloride.